1. Technical Field
The present invention is generally directed toward dendrimers having a core, branches and periphery groups, and more specifically, towards monodispersed dendrimers and method of synthesizing the dendrimers by exponentially increasing the number of branches from the core to the periphery end and exponentially increasing the length of the branches from the periphery end to the core.
2. Related Art
Defect-free synthesis of macromolecules remains a challenge in chemistry, especially for dendrimers, which are finding increased applications in chemistry, materials science, nanotechnology, as well as medicine and pharmacy.(1-7) Dendrimers are tree-like molecules composed of a core (“trunk”), several interior layers (“branches”), and a periphery (“leaves”).(8, 9) However, conventional dendrimer design grows dendrimers disproportionately, that being, the number of branches grows exponentially, but the length of branches remains unchanged. The length of branches refers to the number of covalent bonds connecting adjacent branching nodes. Such an unbalanced growth pattern eventually leads to steric congestion and defective dendrimers.(10-14).
Dendrimers reported in the literature have been obtained by two different synthetic approaches: a) divergent synthesis; b) convergent synthesis. The synthesis of most dendrimers has been accomplished using the divergent process. This implies that a polyfunctional molecule is used as a “core” and that, in order to introduce multiplicity, each functional group is bonded to a molecule which also comprises more than one protected reactive site (“propagation monomer”). A first generation dendrimer is thus formed which, by exhaustive addition of polyfunctionalized monomers, gives rise to the next generation and so on. However, monomer protection/deprotection systems need to be used in order to perform the selective modification of specific groups at each synthetic step.
Convergent synthesis, as first proposed by Frechet (8), differs from the divergent approach in that growth starts at what will become the periphery of the macromolecule. Such a method results in the formation of large dendrimeric fragments, which ultimately are attached through a reactive group (“focal point”) to a polyfunctional “core”. Convergent synthesis has certain advantages over divergent synthesis. With divergent synthesis, the molecule's growth occurs through the simultaneous addition of an increasing number of reactive sites. With the convergent approach, on the other hand, size increase involves a limited number of reactive sites. Convergent synthesis makes use of a smaller excess of reagents. Possible side reactions are therefore avoided and the final products more easily purified.
However, one limitation of the convergent approach is that, as the size of the dendrimers increases, there is an increase in the steric hindrance near the functional group, or focal point, which prevents the group from reacting with the “core.” This limitation is also common in divergent synthesis since the size of the molecule increases more slowly than the number of external functional groups. This leads to an increase in steric hindrance around the functional groups which are thus prevented from reacting to give the next generation.
Thus, to overcome the shortcomings of previous and conventional dendrimer synthesis methods including both convergent and divergent, it would be advantageous to provide for dendrimers that avoid the steric congestion caused during the growth of the dendrimer.